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7

LIQUID-LIQUID EXTRACTION IN HYDROMETALLURGY

the aąueous phase and thus a reasonable water solubility of the extractant. This conflicts with the requirements for an extractant listed above, and so must be considered as representing one limiting case for a broad spectrum of possible models. The other limiting case is the situation where the extract-ant is completely water insoluble and the reactions occur solely at the aąueous-organic interface. In this situation, the partition eąuilibria, repre-sented by eqns (1.5) and (1.8), should be replaced by the appropriate adsorp-tion isotherms relating interfacial to bulk phase concentrations. However, in most circumstances the former scheme, in spite of the obvious limitations, has advantages in describing the extraction chemistry. In addition the equili-brium constants should involve activities of the species present rather thąn . concentrations. However, it is extremely unlikely that such activity data are available. In the laboratory this problem can be circumvented if the concentrations are Iow, sińce activity and concentration may then be regarded as being numerically identical. It should be remembered in practical situations, such as in leach liquors where salt concentrations can be high, substantial differences exist between activity and concentration (Gokcen 1982; Muir and Parker 1982). Although it may be possible in a particular study to circumvent the problem by maintaining a constant ionic strength, it is important to realize that errors can arise in practical hydrometallurgical applications, when using data from laboratory studies at Iow metal and salt concentrations. This caveat is true for all types of extractant.

1.2.1.1 Types of acid extractant

These extractants can be further subdivided into chelating and non-chelating species. The latter include carboxylic, phosphoric, and sulphonic acids (Table 1.1) and provide a very versatile series of reagents. The extent of extraction follows a combination of metal ion basicity and extractant acidity, so that for any particular extractant a selectivity series of elements can be constructed which tends, in the absence of any specific metal-extractant interaction, to follow the lyotropic series.

However, because these reagents are so versatile and extract so many metals their use in hydrometallurgical flowsheets is minimal. Thus although a considerable amount of data have been published on carboxylic acid exiractions (Yamada and Tanaka 1985; Rice 1978; Flett and Jaycock 1973) no current industrial application is known. Similarly, with organosulphonic acids. here the extractants are morę acidic than either the carboxylic or organophosphoric acids, which tends to shift the pH rangę for extraction to lower values that may make integration with the rest of the flowsheet dif-ficult. The condensing of the overa!l pH rangę for metal extraction also makes selectivity for a particular metal a problem (Markovits and Choppin 1973). Finally, both the carboxylic and organosulphonic acids do tend to